The manufacture of plastic articles typically involves the use of polymer resins, solvents, and/or fillers to form materials which are molded into solid articles. There are many ways to process and produce plastic parts. The specifications and end-use requirements for a particular part dictate which manufacturing process is used. Injection, extrusion, and compression molding are currently used to process bio-based plastics. These processes require that the physical form of the feed material be solid particles (such as pellets or powder) or a liquid (molten) feed. Alternatively, solid plastic materials are ground or pulverized prior to processing. An extruder is used to form solid pellets or continuous ribbons by hot-melt compounding the polymer or plastic through shear and heater bands on the barrel. These solid pellets or continues ribbons are then used to form shaped articles. Because long heating cycles and high molding temperatures are required, there is an increased risk of thermal degradation of the polymer occurring during molding. For bio-based polymers, this risk is increased because bio-based polymers typically lack the heat resistance needed to withstand the high processing temperatures seen in injection or compression molding.
Chris Rauwendaal, “SPC Statistical Process Control in Injection Molding and Extrusion,” Hanser Publications, Munich (2000), p. 4-5, describes various drawbacks associated with injection molding as including the expense of the molds, high pressures being generated within the injection molding machine, and the size of the articles being limited to no more than 1 m2. In addition, the thickness of injection molded parts is limited from 0.5 mm to 5 mm. The upper thickness limit is dictated by what is considered to be a reasonable cooling time where the lower limit is set to prevent filling problems resulting from premature solidification.
Woerdeman (US Patent Application Publication No. 2006/0027941A1) sets forth the problem encountered with respect to the low temperature processing of biodegradable articles from wheat gluten dough. In particular, she addresses the problem of solvent removal by subjecting the shaped articles to an environment sufficient to remove excess water such that they are dried into solid biodegradable articles. In particular, she emphasizes the need to strike a balance between the drying of the outer portion of the shaped article and diffusion of water from the inside of the shaped article. This balance is achieved by employing low temperature, controlling the humidity in the environment, or both low temperature and low humidity conditions. If this balance is not achieved, a hard plastic shell co-existing with an uncured center portion occurs, possibly resulting in failed parts. Woerdeman's compression molding process is not suitable when short molding times are required because the dough must be cycled between the mold portions until it is dry to touch.
Yasui et al. in “Gluten Plastic, Biodegradable,” Polymeric Materials Encyclopedia, Volume 4, pp. 2830-2833, 1996; describe the plasticization and molding of gluten. In particular, they teach that a blending temperature is preferably below 40° C. In their process, mastication takes place at 30° C. for 5 minutes. Next, the material is dried at 40° C. for 24 hours and then press molded at 50-150 kgf/cm2 for 20 minutes at 120° C. resulting in a sheet film of gluten plastic. In their process, when the gluten compound was plasticized only by water and the water removed using an oven and compression molding at 130° C. for 4 minutes, a hard and brittle sample resulted that showed the independence of its elastic modulus on temperature. They found that the elastic modulus for a gluten molecule plasticized by polyol instead of water had a rubbery region at room temperature to 80° C.
Other processes typically involve high temperatures and pressures that result in modification of the chemical nature of the materials. For example, Bassi et al. (U.S. Pat. No. 5,665,152) propose the use of injection molding for a method of forming solid, non-edible biodegradable, grain protein-based articles. However, the processing temperature of up to about 80° C. results in essentially complete protein denaturation, resulting in enhanced resistance to biodegradation. Unfortunately, there is no mention of how the solvent is released from the mold during the injection process. Rayas et al. (U.S. Pat. No. 6,045,868) disclose a method wherein grain flour proteins are crosslinked with aldehydes and bleached with a bleaching agent to form crosslinked transparent polymers used as films for packaging. A heating process is preferred in order to concentrate the film-forming solution and denature the flour protein prior to crosslinking and bleaching so that more protein interactions occur when the film is dried and stronger films are formed. In particular, the preferred heating range is 60° C. up to the boiling point of the solvent. Aung (U.S. Pat. No. 5,279,658) describes a pressure injection method where hot dough is pressure injected into a form press having a water-cooled die mold. The expanded hot dough fast cools in the form press at the surface of the cold die mold. The expanded packaging material is stamped to the correct thickness in the form press. The formed packaging material may then be coated with a water repellant material, dried in an oven and cooled in a cooling chamber.
Jane et al. in U.S. Pat. No. 5,523,293 provide a thermoplastic material made of soybean protein combined with a reducing agent, a starch filler, plasticizer, water, and optional additives. The composition may be used for making solid, molded articles that are biodegradable and possess a high degree of tensile strength and water resistance. The articles are made by extrusion compounding or injection molding. More specifically, the composition is made of the reaction product of about 25-65 weight percent soybean protein alone or combined with gluten or other protein, about 0.5-2.5 weight percent reducing agent such as sodium sulfite or sodium bisulfite, about 30-40 weight percent starch filler; about 5-35 weight percent glycerol or other plasticizer, and about 5-25 weight percent water. The composition may include additives, preservatives, and/or coloring agents. The compositions are prepared by high speed, high shear mixing at an elevated temperature to melt the protein mixture, with extrusion processing being preferred. The dried extrudate is processed, for example, by injection molding, to make solid, molded articles that are biodegradable and have a high degree of tensile strength and water resistance. The high speed, high shear mixing at elevated temperatures is not desired in the present invention as such activity results in shear thinning of the material and a reduction of chain entanglements, causing the resulting articles to be brittle and weak.
In addition to injection, extrusion, and compression molding techniques, centrifugal molding is also considered to be a suitable molding technique for various materials. Centrifugal molding techniques are generally known in the art but are far less common in the plastics industry. When centrifugal molding takes place, the mold or molds in which the parts are formed are rotated at high speed and the substances of different densities within the mold or molds are separated by centrifugal force.
An object of the present invention is to provide a process for preparing biodegradable articles from a biodegradable material.
Another object of the present invention is to provide a process for preparing biodegradable articles which employs at least one mold which is subjected to a centrifugal force in order to separate solvent from the biodegradable material as the article is molded.